linux-kernel - Re: [PATCH v2 2/3] tun: Pad virtio header with zero

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Message-ID: <CACGkMEs73Pms5FB3ouzrLsDjAsQ4OhMMDVD2LnO6kVHCsN0A0w@mail.gmail.com>
Date: Fri, 10 Jan 2025 11:27:23 +0800
From: Jason Wang <jasowang@...hat.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
Cc: Jonathan Corbet <corbet@....net>, Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, kvm@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org, 
	Yuri Benditovich <yuri.benditovich@...nix.com>, Andrew Melnychenko <andrew@...nix.com>, 
	Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com, devel@...nix.com
Subject: Re: [PATCH v2 2/3] tun: Pad virtio header with zero

On Thu, Jan 9, 2025 at 2:59 PM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>
> tun used to simply advance iov_iter when it needs to pad virtio header,
> which leaves the garbage in the buffer as is. This is especially
> problematic when tun starts to allow enabling the hash reporting
> feature; even if the feature is enabled, the packet may lack a hash
> value and may contain a hole in the virtio header because the packet
> arrived before the feature gets enabled or does not contain the
> header fields to be hashed. If the hole is not filled with zero, it is
> impossible to tell if the packet lacks a hash value.

I'm not sure I will get here, could we do this in the series of hash reporting?

>
> In theory, a user of tun can fill the buffer with zero before calling
> read() to avoid such a problem, but leaving the garbage in the buffer is
> awkward anyway so fill the buffer in tun.
>
> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
> ---
>  drivers/net/tun_vnet.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c
> index fe842df9e9ef..ffb2186facd3 100644
> --- a/drivers/net/tun_vnet.c
> +++ b/drivers/net/tun_vnet.c
> @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
>         if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr))
>                 return -EFAULT;
>
> -       iov_iter_advance(iter, sz - sizeof(*hdr));
> +       if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr))
> +               return -EFAULT;
>
>         return 0;

There're various callers of iov_iter_advance(), do we need to fix them all?

Thanks

>  }

>
> --
> 2.47.1
>